Brass



Feb. l2, 1946.

G. H. EDMUNDS BRASS A Filed Feb. 11, 1945 3 Sheets-Sheet 1 soooA loo 20oTTCRNEY l Feb. 1'2, 1946. G+.. EDMUNDS 2,394,673 y BRAS S Filed Feb. 11,1943 3`Sheets-Sheet 2 ATToRNEYs Feb. 12, 1,946. G, H; EDMUNDS 2,394,673

BRASS l Filed Feb. l1, 1943 S'Sheets-Sheet .'5

4 ATTORNEYS cracking.

` Patented 12, l I

lassists` 'Y naass l Gerald E. Edmunds. Palmerton, Pa., assignor to 'TheNew Jersey IZinc Company;` New York, y N. Y., a corporation of NewJersey 'Application February 11, 194s, serial No; I475,502 s claims.(ci. 'z5-151.5)

This invention is concerned with Vbrass and provides a method forinhibiting season cracking in brass as well as a novel brass compositionthat has high resistance to season cracking.

lSeason" cracking is a well known phenomenon corrosive environments.Thus, exposure of internally stressed brassto the action of liquid metalsuch as mercury ormolten tin or to the action and tends to developin'brasses, especially brasses ,containing 60 to 85%'or even morecopper, when lthe brass is exposed in a state of stress to certain of anitrogencompound, for example.- an amine,

an ammonia compound or anoxide of nitrogen tends to cause the brass todevelop a seriesof fine cracks, which greatlyj impair its strength andusefulness. Season crackingisparticularly trousmokelesspowder `and thefulmlnate'tend to decomposeto form compounds that promote seasonordinarily contain in the neighborhood of 70% 'copper and l8,0% zinc,but has application to all brasses which tend to fail through seasoncracking.

I have discovered that the resistance of brass to seasonV cracking canbe increased tremendously f by incorporating in the brass from about0.9%

to about 1.3% siliconand thereafter heat-treating the brass at atemperature ranging from '125 C.

melting of the brass. Accordingly, a brass of the invention consists offrom 60% to 85%' of'copper,

from 0.9% to 1.3% of'lsilicon'with the remainder l zinc. However.. the"high resistance to season cracking thus acquired is, in large/part, lostif' the brass thereafter is subjected to heat treat-v ment at atemperature substantially below '725 C.

Consequently, in the practice of my vinvention the last heat treatmentmust be conducted at a temperature in excessof 725 4C., although priorannealing treatments and the like may be contreatment at a temperatureof 725 C. or highery with, a quick cooling operation. for example byquenching in water or in a current of-'cool air.

Thel invention is, therefore, particularly useful in the caserofcartridgebrasses, which 'blesome in Athecase of cartridge brass whichcomes incontact with smokeless powder or a mercury compound such as-fulminata for both Such quick cooling increases the resistance ofsilicon brass to season cracking and, as already noted, thisresistance'will 4be retained even after- V subsequent and severe coldworking.

It will b e recognized-that they final heat treatment of the brass 1naccordance with my invention is conducted at temperatures considerablyin excess of those customarily employed in brass annealing practice. invwhich stress relief or recrystallization is brought about byanneallngin a rangeof about 250 C. to about 650 C.

In accordance with my invention, the requisite heattreatment andAquickly\coo1ing of the brass containing the silicon can be conducted atany stage in a brass fabricating operation, provided that following theheat treatment at theihigh temperature the brass is not subjected tosubsequent heat treatment at a temperature below 725. C. and above thatemployed in baking vorganiofinishesv and the like, say 175 C. In otherwords, the relatively moderate heat treatment involved in baking organicfinishes may'not be injurious.

, 'The practice of my inventions described y above. i.' e. a hightemperature last heat'treatment of a silicon-bearing brass (containing40.9% tol 1.3% Si) preferably followed by quick cooling, brings abouttheformation of a brass of novel structure which, in general, comprisesalpha brass grains (with or without' beta grassv grains), toe

'gether with an additional microstructuralconto the solidus, i. e."th'etemperature lof initial stituent present aty the alpha grain boundaries,J

which` constituent upon heat treatment for one hour at'375 C. willdecompose into a plurality of other microstructural constituents thatare visible in a polished section at a magnification of 1000 diameters,atleast one of said plurality of constituents being other than' alphaand beta' brassesand harder than either. A certain amount ofdecomposition'of this additional microstructural constituent may betolerated without great l sacrifice of resistance to season cracking.Consequently, the brass made in accordance with proportion of theadditional microstructural con- I have found that the Vadditionalmiorostruc tural' constituent identified above is always present insilicon brasses which manifest adequate resistance to season cracking,arid the presence of this constituent is a reliable index as to whetherVor not a given silicon-brass has the desired quality. The chemicalanalysis of the additional microstructural constituent has not beendetermined, but the constituent is easily recognized because oflitsproperty of decomposing upon mild heat treatment, (say any annealingconducted at a temperature much below 725 C to form the plurality ofother microstructural .constituents at least one of which is harder thanalpha or beta brass. Thus, upon decomposition; the second additionalmicrostructural constituent may develop a fibrous structure (principallyadjacent thel alpha grain boundaries) in which some of theiibers arevery hard. On the other hand, the additional microstructural constituentmay decompose to give a mottledvstructure in which hard spots, arerecognizable. It should be borne in mind that prolonged annealing` attemperatures below 725 C., say annealing for 1 hour at 500 ',C., resultsin the complete disappearance of the, additional microstructuralconstituents.

Another important characteristic of the brass of the invention residesin its small grain size, which does not exceed 0.1 millimeter, i. e. theaverage grain diameter is below this figure.I

In-general, the additional*microstructural coni 30 the stituent tends tooccur at the alpha grain boundaries and in many instances substantiallyenvelopes the alpha grains. -In addition, the second microstructuralconstituentimay occur within the grains, either alpha or beta. n

The application of my invention to the manufacture of brass cartridgesissimple. Ordinarily, such cartridges have been -made of high grade brasscontaining about '70% copper and 30% zinc without other alloyingingredients. This brass `is worked into a body through several stages,with an an'nealing atabout 600 C. following each stage. After the lastbody annealing, it is cus- ,tomary to anneal the neck portion of thecase prior to tapering it to bullet diameter and after tapering to applya recrystallizing anneal to the mouth. In the practice of vmy inventionthe lonly required modiilcation of the foregoing is to include therequired proportion of silicon (i. e..0.9 Ato 1.3%) in the initial alloyand to conduct the with and-without silicon; and

25 ples wer 2,siw.,oval

A script panying drawings in which Fig. 1 is a graph illustratingresistance to'season cracking of silicon brass in its relation toannealing temperatures.

ion, taken in conjunction with the accom- Fig. 2 is a graph illustratingthe effect of annealing temperature onV the grain size of brass Figs.3,4, 5 and 6 are photomicrographs illustrating the effect of variousannealing treatments ion. the microstructure of a silicon brass.

"Generally speaking, the higher the annealing temperature above theminimum of '725 C., the

greater is the resistance of the final product to season cracking. Thisis shown in Fig. 1, which isa graph of results obtained with coinmercial*I0-30 cartridge brass and with a brass of similar composition exceptthat it contained 1.2% silicon. Both kinds-of brass were annealed forone- 'half hour in nitrogen at various temperatures and water quenched.Thereafter all the samples were subjected. to a stress-cracking orseasoncracking test in an atmosphere of ammonia;4 carbon dioxide, airand water vapor while thev same under a tensile stress of 15,000 lbs.per square inch. On Fig. l, minutes is plottedona logarithmic scaleagainst annealing temperatures in degrees centigrade.

Referring to Fig. 1, it will be observed that as rolled was about 500(minutes to failure) This as increased to a maximum of .about 620 byannealing at 300 to 400 C. Higher annealing temperatures .brought abouta continued decrease in 35 the resistance of the conventional'brass toseason cracking, so .that when the material was. an-

' nealed at 800 C. it had a resistance of only 130.

The brass containing 1.2% silicon had a higher` initial resistance;(1500) tov season cracking.v This 40 resistance was decreased slightlyby annealing temperatures up to about 550 C. and at the lattertemperature had a resistance of about 900. There was a slight increasefrom the 900 figure a's annealing temperatures were increased to 700 C.,at which point the resistance to season cracking was about 1000. Above'700 C. however,`

the resistance to -season cracking was. increased markedly. Thus,annealing at '150 C. increased Iii) last body anneal at a hightemperature, say-be# tween 725. C. and 870 C.'the` approximate solidusfor 'l0-30 brass containing 1% silicon.

' The last body annealing should-be followed by the season crackingresistance to almost 4000 and this was increased toA 7000 when theannealing was conducted at 800 C. In short, it was pos-` sible' inaccordance with the practice of the invetion to obtain a resistance toseason cracking which was more than ten times that developed inconventional cartridge brass.

I have found that the presence of silicon withiii the range of 0.9% to1.3% has a highly favor- The annealing treatments conductedon theV neckand mouth portions of the cartridge case subsequent^to the nal bodyannealing do not deleteriously inuence thecartridge cases or detractfrom t-he value of the invention as applied to the fabrication of suchproducts, because season-cracking failures are less serious at thefneckand mouth portions of a cartridge case than at the head end of thecartridge case, where the ltwo l anneals (neck and 'mouth) have littleeffect. Moreover; season cracking at neck and mouth are avoidable bycareful manufacture. ,Hence in the head the structure that manifests ihigh resiste-" ance to'season cracking (an inthe iinal body anneal atthe higher 'temperad which yis-developedi ture) is preserved. A

My linventi n will be stood inthelight ofthe following detailed .de

more thoroughly undrable effect upon the microstructure of the brass inthat it tends to inhibit grain growth brought about byrecrystallization. This is illustrated in Fig. 2 which shows the effectof annealing at .various temperatures followed by water quenching uponthe conventional cartridge brass of 'Figf 2 and upon achemically similarcartridge brass except that it contains 1.2%- silicon. It

will be observed that in the case 4of conventional cartridge brass, highannealing temperatures tend to develop a structure that is far toocoarse j and not tolerated in'l military specifications. Thus, annealingat 600 C. brings about the formation of grains-having an averagediameter of about .08 millimeter. The 'grain'sizeis increased to.2millimeterby annealing at '150 C. Since a gram size in excess of 0.2millimeter is the time to failure in V resistance of the conventionalcartridge brass highly undesirable, it is'appar'eni that high temingrain size.

.lowed by air cooling;

1 4perature annealing cannot be' errililoydA in the vcase oiconventional cartridge brasses,

In contrast with the effect of high tempera- `ture annealing uponconventional cartridge brasses, Fig. 2 shows that in the case of thesilil con brasses of .my invetnion, high temperature annealing does notbring about excessive increase Up to a temperaturen! 450 C., no increasein grain size occurs. Between 450C. and 700 C., the brass recrystallizeswith an in "crease in grain size to about .04 mm. and thereafter thecurve is substantially ilat. the maximum grain size obtained being .04millimeter. YThis is ,well within the tolerance established by militaryspeciiications.

The cracks developed in the foregoing te ts and in parallel testsemploying mercury as e stress-cracking agent were subjected to micro'-scopic examination. It was found that thepath of vfailure inconventional cartridge lbrass wasintercrystalline whether thestressf-cracking agent were ammonia or mercury. However, in the Isilicon alloys, the path oi failure was intercrystalline when the stresscracking agent was mercury but generally transcrystalline in the ammoniastress cracking test.

The microstructure oi alloys made in accordance withv my invention willbe more thoroughly understood inthe light of Figs. 3 to 6,-which .arephoton'iicrogr'aph-s oi 7030 brass containing 1.2 silicon and subjectedto various annealing procedures. In all cases the brass contained 69.8%

' copper, 1.2% silicon, 0.005% iron. less than 0.01% lead and thebalance substantially all zinc. This alloy was cast as a`1'.' by 31/2 by10" slab.

ing series of operations:

`Thereafter the slab was sublectedgto thefollow- 12.` Cold rollin-g toy0.045 thickness. y f

Fig. 3 represents the structure ofthe brass fol- .lowing annealing for30'minutes at 625 C. fol- The magnification is ap proximat'ely 75diameters and the etchantl emr ployed prior to photographing thepolished sec tion comprised 5 parts Ioi' ammonium hydroxide to 1 parthydrogen peroxide'. f The structurezshown is similar to that obtained inordinary commercial brass annealing and lacks grain boundaryconstituents. In other words, Fig. 3 illustrates the structureobtained'in' annealing silicon brasses below 725 C. (without priorannealing at temperatures above 725 CJ' and `is not greatly differentfrom that obtained in conventional annealingoi ordinary brass.

Fig. 4 represents. the structure obtained when lthesilicon brass wasannealed for 1A hr. at 800 C.\ ioll`oweiby water quenching Themagnification in Fig. 4 is approximately 250diameters and 2,394,673 l l3 undecomposed additional constituent at the 'grain boundaries.

Figs. 5 and 6 are photomici'ographs oi a part of the specimen of Fig.'which was subjected to 5 a further anneal.

Thus, this part, -following the annealing at 800 C. and the quenching inwater,A was reannealed for 1 hr.l at 375 C. and air cooled. Thereannealing and the slow cooling brought about a l decomposition of theadditional constituent at the grain boundaries with the resultantformation'of the new-constituent. which is harder than v both alpha andbeta brass. Fig. shows the structure at 250 magniilcations and Fig. 6shows lo the structure at 1000 magniflcations.

In Fig. `6 the hard constituent, resulting from the decomposition of theconstituent shown in thel grain boundaries of Fig. 3, is particularlyapparent. It is represented by the dark lines and dots on thephotomicrograph.

Iclaim:

' 1. 'A brass having high resistance to'season cracking consistingessentiallyoi zinc, copperand 0.9% to 1.3% silicon and comprising .alphabrass grains together with an additional microstructural constituentpresent at the alpha grairrboundaries, which constituent is formed byheat treatment at a temperature ranging from 725 C. to the solidus andupon heat treatment for one hour 'at 375 C. is destroyed bydecomposition .into

a plurality oi'. otherv microstructural constituents that are visible ina polished `section at-a magnication of lodiameters, at least one ofsaid plurality of constituents'being other than alpha and beta brasses.

2. A brass having high resistance to season cracking consistingessentially oi' zinc, copper and 0.9% to 1.3% silicon and comprisingalpha brass grains together4 with an additional' microi 40 structuralconstituent and a;plurality or other microstructural constituentsresulting from decompositionof a portion of the additional constituentduring heat treatment of the brass at moderate temperatures, at leastone oi' said plu- 5 rality of constituents being harder than valpha andbeta brasses and visible in a polished section at a magnification oi'1000 diameters.

` 3. A brass having high resistance, to season vcracking consistingessentially o! zinc, copper and 0.9%. to 1.3% silicon and comprisingalpha brass grains together with an, additional microstructuralconstituent present at the alpha grain boundaries, which constituent isformed by heat treatment at-a temperatureranging from 725 C.tothejsolidus and upon heat .treatment for one hour at 375 -C.` isdestroyedv by decomposition -into a plurality of other microstructuralconstituents that are visible in a polished section at a magnilcation of1000 diameters, vatleast one of 5 0 said plurality of constituents beingother than alpha `and beta Abrasses, the average grain diameter of thebrass beingl lessthan 0.1 milli- Jmeter. f,

4. n -rolled brass article having high resistance '05 to season^crackingconsisting ot from 60% to 85% of copper, from 0.9% to 1.3% of siliconwith the remainder substantially all zinc, said brass comprisingalphagbrass' grains and having been s'ubjected to a final' heattreatment at a temperature .70` in the range from 725 C. to'the solidusand the .etchant employed on the' polished-section was -10-grams ofchromic 'acid in 100 c c. oiyater which 5 drops'oi.' hydrochloric acidIl ibldbeen added. Fig.`4 illustrates thel'fmicro-structure oi' thealloys of my inventionand shows fthe clear'. 7i about 0.9% to1 1.3%siliconfand the remainder cooled quickly. v v

brass, the improvement which comprises forming the cases from brasscontaining about 70% copper,

'c vzinciccnaucuncne: last-'amieaung step at a. temperatureof from 125C. tothe soliduaand.

quickly cooling the cases. from said temperature range.

6. In the'fabrication of articles from brass, the

. improvementI which comprises. forming thel remainder zinc,vmechanicallfl working the brass. giving the mechanicallyworked articlesa final heat treatment at a temperature of from '725 C;

to the soiidus, and quickly cooling the brass from 5 said temperaturerange, whereby the resistance o! .the brassto season cracking isenhanced.

GERALD

